1. Field of the Invention
The present invention relates to an improved process for preparing 2-cyano-3,3-diarylacrylic esters of the general formula I ##STR4## where R.sup.1 and R.sup.2 are hydrogen, C.sub.1 -C.sub.4 -alkyl groups, C.sub.1 -C.sub.4 -alkoxy groups or di(C.sub.1 -C.sub.4 -alkyl)amino groups and R.sup.3 is a C.sub.4 -C.sub.18 -alkyl group which can be interrupted by ether-functional oxygen atoms, by reacting the appropriate benzophenone imines with the appropriate 2-cyanoacetic esters.
2-Cyano-3,3-diarylacrylic esters are highly effective UV absorbers which are used in particular as light stabilizers in plastics and cosmetic products.
2. Description of the Background
It is generally known to prepare compounds of type I from the benzophenones by reaction with cyanoacetic esters (see, for example, EP-A1 430 023, US-A 3 215 724 and DE-Al 43 14 035).
Although this reaction requires the relatively high temperatures of from 70 to 130.degree. C., it takes place only slowly and therefore requires the use of a catalyst, which must subsequently be removed from the product. In addition, unwanted concomitant substances are produced and, for this reason, elaborate purification steps must follow the reaction.
It is furthermore known (Bull. Chem. Soc. Fr. (1963) 1576 (G. Charles)) to prepare ethyl 2-cyano-3,3-diphenylacrylate from benzophenone imine and ethyl 2-cyanoacetate at 70 to 120.degree. C., but this reaction has the disadvantage that colored byproducts are produced at the reaction temperatures, and the liberated ammonia reacts to an interfering extent with the ester moiety of the molecule to give the corresponding amide ##STR5## ie. elaborate purification stages are also necessary in this case, especially in order to make the products suitable for use in cosmetic products.